X-ray fluorescence is a powerful technique for material composition analysis in many industrial and research applications. X-ray fluorescence results when an electron makes a transition from a high atomic orbit to a lower one. With sufficient energy, the excitation radiation, such as x rays, energetic electrons, or ions, eject inner shell electrons by means of photo-electric effect to create a vacancy in the inner shell. This effect results in an unstable atomic state, and the outer shell electrons can transfer to this lower energy shell by a radiative process.
FIG. 1 illustrates the radiative process with respect to the K (left) and L (right) line emissions from a titanium atom. Left: a vacant K shell can be filled by an electron making a transition from L or M shells, resulting the Kα and Kβ line emissions, respectively. Right: a vacant L shell can be filled by an electron making a transition from M or N shells, resulting in the Lα and Lβ line emissions, respectively.
The fluorescence x rays have a characteristic energy that is equal to the difference between the two atomic shells, and is often called an emission line. It is characteristic of the particular element. One or a series of emission lines can be used to uniquely identify an element. Transitions to fill a vacancy in K shell results in K-line emission, which is further distinguished by the transition from L shell as Kα, and M shell as Kβ, etc. Analogously, transitions to L-shell results in L-line fluorescence emissions. Generally, the transition from the nearest shell has the highest probability and therefore the α lines typically have higher intensity.
FIG. 2A shows x-ray K-shell and L-shell fluorescence emission energy, and FIG. 2B shows the yield as a function of atomic number, i.e., for different elements. From these plots, it is shown that the fluorescence yield generally increases as a function of the atomic number. The K-shell yield is about 1% for elements in organic compounds, C, H, and, O. It increases to about 5% for higher Z elements such as Na, Mg, and P. The yield of L-shell emissions is generally lower than with the K-shell. A few percent can be expected for the middle Z elements such as Mn, Fe, Cu, and Zn.
Besides x-ray excitation, x-ray fluorescence can also be generated by other energetic beams such as ions or electron. X-ray fluorescence from electron excitation is commonly measured with energy-dispersive spectroscopy (EDS) detectors in scanning electron microscopes (SEM) to analyze the material composition of a sample. In addition to the characteristic emission lines, the emission spectra from electron excitation also contain a broad continuum called the Bremstrahlung radiation.
FIG. 3 is a plot of showing the level of the Bremstrahlung radiation relative to the radiation from the emission lines for emission spectrum of bulk titanium excited by 10 kilo electron-Volt electrons. The integrated intensity of the continuum is of similar magnitude to emission lines.
The presence of this background Bremstrahlung radiation makes it extremely difficult to detect minor constituents and trace elements in a sample since their signal is usually buried in the continuum. Consequently electron excitation is generally only used to measure composition with relatively high concentrations of higher than 1%.
The spatial resolution of the EDS technique is limited to about 1 micrometer (μm) by electron scattering inside the sample. In contrast, emission generated by x-ray excitation contains negligible amount of Bremstrahlung background, and consequently, much higher material sensitivity can be achieved: trace element identification with parts per million (ppm) concentrations have been demonstrated.
Current microprobes that “focus” the excitation beam include the focused electron beam (such as in SEM) and the x-ray beam focused by Fresnel zone plates, reflective optics, and near-field capillary optics. The x-ray emitted from the sample is then detected by an EDS or wavelength dispersive spectrometry (WDS) detector, or an x-ray imaging system.
In x-ray imaging systems, the exciting radiation typically comes from a synchrotron radiation source or a rotating anode laboratory source. In many synchrotron radiation source systems, the x-rays are focused to the sample by a zone plate lens. The focused x-ray beam excites x-ray fluorescence emission, which is detected by the EDS detector. In laboratory source systems, the x-ray microprobe consists of an x-ray tube and an x-ray lens (Fresnel zone plate or capillary optics). The probe excites x-ray emission from the sample, which is imaged to a detector by an objective lens (Fresnel zone plate). Note that the spatial resolution of the synchrotron source systems depends entirely on that of the microprobe, in the laboratory source systems, the spatial resolution depends on both the microprobe and the imaging system.